1. Cross Reference to a Related Application:
A related application entitled LASER MODE LOCKING METHOD AND APPARATUS, U.S. Ser. No. 317,207, to Daniel J. McGraw, is being filed concurrently herewith, and the specification thereof is incorporated herein by reference.
2. Field of the Invention:
The invention relates to a method and apparatus for locking lasers using passive and active systems.
3. Description of the Related Art:
Various lasers have been mode locked in a number of ways in the past. Recently it has become possible to mode lock solid state lasers such as Neodymium: Yttrium Aluminum Garnet (Nd:YAG) lasers, as disclosed in "A Mirror with Intensity--Dependent Reflection Coefficient" by K. A. Stankov (1986-87). This paper describes how a nonlinear optical crystal-mirror combination acts like a mirror whose reflectivity at the fundamental frequency increases with intensity. The second harmonic generation crystal is followed by a mirror that has a low reflectivity (R.sub.1) at the fundamental frequency and a high reflectivity (R.sub.2) at the second harmonic. The distance between the crystal and the end mirror is adjusted so that the dispersion of air creates a half wave phase shift between the fundamental and second harmonic. This phase shift, as was shown in J. M. Yarborough, J. Falk, C. B. Hitz, Applied Phys. Lett. Vol. 18, p. 70 (1970), causes a reverse in the direction of energy transfer on the second pass through the crystal, and the second harmonic is converted back to the fundamental. Thus, as the intensity at the fundamental frequency increases, starting at zero, the conversion efficiency to the second harmonic frequency increases from zero to near 100%, which causes the reflectivity of the crystal mirror combination to increase from R.sub.1 to a value near R.sub.2.
"A New Mode Locking Technique Using A Non-linear Mirror" by K. A. Stankov and J. Jethwa (1986-88) and "A Novel Nonlinear Optical Device for Passive Mode Locking" by K. A. Stankov (Apr. 1988) each recognize that a mirror whose reflectivity increases with intensity can be used to provide passive mode locking. Stankov demonstrates that it is possible to passively mode lock a high peak power Q-switched Nd:YAG laser using a crystal and mirror combination. However, this structure of Stankov's nonlinear mirror precludes its use in mode locking low average power lasers such as continuous wave Nd:YAG and diode lasers. The basic problem with Stankov's design is that in order to achieve mode locking, there must be a significantly large intensity dependent modulation of the reflectivity (.DELTA.R) of the mirror. However, two conditions that must be simultaneously maintained to achieve a large AR are mutually exclusive conditions in the context of Stankov's design. Both conditions cannot be simultaneously satisfied and in the Stankov design the failure to satisfy either condition prevents mode locking in all except very high power lasers. The two conditions required for large .DELTA. R are (a) that R.sub.1 be much less than R.sub.2 ; and (b) that a high intra-cavity power at the fundamental frequency be maintained to generate a significant conversion efficiency to the second harmonic which produces the large .DELTA.R. Condition (a) requires a high output coupling for the fundamental frequency and so prevents a large buildup of the intra-cavity power at the fundamental frequency. Similarly, satisfying condition (b) requires violating condition (a).
Essentially, Stankov mode locks a laser by using intra-cavity second harmonic generation to generate an effective cavity mirror, the reflectivity of which is an increasing function of the intensity at the fundamental frequency. This necessitates the use of a relatively high power pumping laser since the reflectivity of the mirror increases roughly proportional to the square of the second harmonic conversion efficiency. Stankov found experimentally that a 2% to 3% continuous wave conversion to the second harmonic was required to cause passive mode locking to occur. The Stankov system will simply not work with moderate or low intensity lasers where typical conversion efficiencies are less than 10.sup.-4.
Although sum frequency generation mode locking in accordance with the invention at first blush may appear similar to Stankov's, closer inspection reveals that it is fundamentally different in that conditions (a) and (b) can be and are simultaneously satisfied when practicing the invention. This is because in practicing the invention the conversion efficiency from a fundamental frequency, e.g., (.lambda..sub.1 =810 nm), to a sum frequency, e.g., (.lambda..sub.3 =460 nm), does not depend on the intensity at the fundamental frequency. The conversion efficiency instead novelly depends on the intensity at the second lasing frequency (.lambda..sub.2 =1064 nm). The two above-mentioned conditions can, in accordance with the invention, then be easily satisfied: (a) R.sub.1 &lt;&lt;R.sub.2 and R.sub.1 &lt;&lt;R.sub.3 ; and (b) high conversion efficiency (yielding high .DELTA.R.sub.1) is achieved by maintaining a high intra-cavity intensity at .lambda..sub.2 =1064 nm. These two conditions can be simultaneously achieved by taking R.sub.2 .perspectiveto.1.0. In practicing the invention, currently available single stripe diode lasers used in a diode pumped Nd:YAG laser can yield a .DELTA.R greater than 10%. This is significantly larger than the 2% to 3% value Stankov found experimentally as the threshold for the onset of passive mode locking.